The present invention relates to a device for the detection of back-scattered electrons emitted by a specimen in an electron microscope, and more particularly to such a device having a converter to convert the back-scattered electrons into secondary electrons and a detector for the secondary electrons.
In an electron microscope, the specimen to be examined is exposed to the impact of an electron beam. In the process, low energy secondary electrons are released by the specimen. Furthermore, a greater fraction of the incident electrons are scattered on the specimen with a slight loss of energy. These back-scattered electrons in turn produce secondary electrons upon their impact on the walls or other parts of the object chamber or during their repeated collision with the specimen, which are detected by a secondary electron detector together with the secondary electrons originally released by the specimen. Of the back-scattered electrons themselves, only those that are scattered in the direction of the detector are detected.
Since the secondary electrons released from the specimen and the back-scattered electrons scattered on the specimen contain different information concerning the specimen, attempts have therefore been made to detect them independently from one another and as completely as possible. An apparatus partially suitable for the purpose has been described in Scanning Electron Microscopy (1978), Volume 1, pages 303 to 310.
In order to separate the secondary electrons from the specimen from the back-scattered electrons, the specimen is provided with a shielding grid. A negative potential applied to the shielding grid retains the low energy secondary electrons. The higher-energy back-scattered electrons pass through the shielding grid and, upon their impact on the walls of the chamber and in particular on the pole piece of the electron optical device adjacent to the specimen, they release secondary electrons. A field of positive potential in front of the secondary electron detector draws these secondary electrons into the detector. It has been proposed in this connection to increase the number of secondary electrons produced by the back-scattered electrons by arranging a plate with a high secondary electron emission coefficient in front of the pole pieces of the electron optical device. Furthermore, a special configuration of this electron conversion plate is intended to insure the detection of as many as possible of the secondary electrons by the detector. Even though this known apparatus makes it possible to separately detect the back-scattered electrons by means of their increased conversion into secondary electrons, it is not feasible to measure the secondary electrons produced on the specimen separately from the back-scattered electrons.
The secondary electrons produced at the specimen may be detected together with the back-scattered electrons, when the shielding grid over the specimen is grounded or has a weak positive potential applied to it. It is found, however, that the information contained in the primary flow of secondary electrons is masked increasingly by the flow of secondary electrons produced by the conversion.